Electrode for electric discharge devices



Nav. 22, 1949 H. A. KLAsENsET AL 2,483,727

ELECTRODE FOR ELECTRIC DISCHARGE DEVICES Filed Jan. 7, 1948 PatentedNov. 22, 1949 ELECTRODE FOR ELECTRIC DISCHARGE DEVICES Hendrik AnneKlasens, Mitcham Junction, and Thomas Holmes, London, England, assignorsto General Electric Company, a corporation of New York ApplicationJanuary 7, 1948, Serial No. 854 In Great Britain January 13, 1947 1Claim.

This invention relates to electric discharge devices and in particularto discharge lamps of the cold starting type filled with gas and/ormetal vapor at a low pressure. When a sumciently high voltage is appliedbetween the electrodes of such a tube the lamp strikes. The cathodelosses are high initially but decrease when the electrodes are heated upby the discharge until a final equilibrium is reached whereby thevoltage drop near the cathode is small but just enough to keep it at therequisite emitting temperature.

It is a disadvantage of the conventional types of electrodes, consistingof a metal wire coated with emitting material, that the coatingdisintegrates during life, especially during the period when theelectrodes are heating up and the cathode losses are high. The life ofthe lamps is shortened by this disintegration. Moreover the sputteredmaterial will be deposited on the glass wall near the electrodes causingan unsightly blackening. It is known that the life of the tube can beimproved by using a solid rod of emitting material enclosed in a wirehelix. The emitting surface still disintegrates but the emission iscontinuously restored by diiiusion of the emitting material, for examplean alkaline earth metal, from the inside to the surface. This does notprevent however the deposition of sputtered material on the glass.

It is an object of our invention to provide an electrode of the coldstarting type which will have a very long life. Other objects are toprovide an electrode which is highly resistant to disintegration, heatsup quickly upon starting of the discharge and requires only a smallvoltage drop to keep it at the requisite temperature during operation,and minimizes or' practically eliminates blackening of the surroundingenvelope.

Other objects and advantages of our invention will appear from thefollowing description and from the drawing which is a somewhatdiagrammatic elevation of one end of a gaseous discharge lamp embodyingan electrode comprising our invention. 7

According to the present invention, in an electric discharge lamp of thetype referred to above and comprising a sealed envelope l filled withgas and/or metal vapor at low pressure, the electrodes 2 are constitutedof hollow bodies made of an alloy of one or more alkali metals oralkaline earth metals with a metal or metal alloy capable of forming asubstantially homogeneous alloy therewith. Preferably the alkaline earthor a nickel-copper alloy. The surface of such emissive alloys can bemade strongly emitting by mild oxidation (e. g., by heating in anatmosphere of oxygen) whereby a thin layer of metal oxide is formed,followed by heating in a vacuum at such temperature that the alkalimetal or alkaline earthdifiuses to the surface. In this way a surfacelayer is formed, comprising inter alia the free alkali metal(s) or thefree alkaline earth metal(s) or a mixture of these metals, together withthe oxide(s) of the said metal(s).

The said alloy may be made by melting together nickel and barium in theabsence of air, e. g., in the atmosphere of argon. To preventevaporation of barium, chromium may be added, and a certain percentageof copper may also be added to increase the solubility of the barium.

A suitable composition is:

Per cent Barium 0.5 Chromium 1.0 Copper 10.0

Nickel, balance.

The material is drawn into tube form, and

the tubes are activated by heating for about 20 seconds in oxygen at 10-mm. pressure, for inlamp the discharge covers only the inside of thetube, i. e., the so-called hollow cathode effect is obtained. In thecase of a discharge lamp filled with argon at about 3 mm. pressure,

an internal diameter of 2 mm. is satisfactory.

It will be understood that each end of the envelope 1 is provided withan electrode 2 which may be supported by a lead-in conductor 3.

According to a further feature of the invention a shield 4 which may bein the form of a plate may be positioned adjacent the open end of theelectrode 2 so as to cover the opening while leaving suflicient spacefor the discharge to pass between the shield and the inside of theelectrode. In this way any blackening of the envelope of the lamp nearthe electrode, which in any case is greatly diminished because thesputterlng takes place only inside the electrode and nearly all thesputtered material is collected on the electrode itself, issubstantially eliminated metal used is barium and itis alloyed withnickel as sput material escaping from e p n 3 A end of the electrodewill be collected on the shield. A slight bombardment of the outside ofthe electrode may still take place and cause sputtering of metal andblackening of the envelope, but this may be prevented by coating theoutside of the electrode with an insulator, such as glass as indicatedby the dotted outline 5.

Electrodes made according to the invention have a very long life, notonly because the emitting layer strongly adheres to the metal carrierbut also because any loss in emitted material is replenished from thebody of the electrode. Another advantage is that heat losses byconvection are low, so that the electrodes are heated up very quickly onstarting and only a small voltage drop is necessary to keep them at therequisite temperature during operation of the lamp.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

In a gaseous electric discharge device, an electrode comprising a hollowbody composed of a substantially homogeneous alloy containing mainlynickel together with approximately 10% copper, 1% chromium and 0.5%barium.

HENDRIK ANNE KLASENS. THOMAS HOLMES.

REFERENCES CITED The following references are of record in the

